1. Field of the Invention
The subject invention concerns automated inspection systems and, more particularly, automated systems for optically inspecting workpieces having concave surfaces or internal cavities as herein described.
2. Description of the Prior Art
Various types of systems for the inspection of workpieces having concave surfaces or internal chambers or cavities are known in the prior art. Examples of such devices include containers, molds, closures, caps, lids and items with concave surfaces as well as other similar structures. Such workpieces with internal cavities or concave surfaces are generally referred to herein as "recessed workpieces" or "workpieces with recessed surfaces."
Some known inspection systems such as illustrated in U.S. Pat. Nos. 4,733,973 and 4,899,573 are based on pressurization or physical measurement such as shown in U.S. Pat. Nos. 5,604,442; 5,558,233 and 5,532,605; and 5,388,707. Other types of inspection systems such as described in U.S. Pat. Nos. 4,791,287; 4,778,999; and 5,365,084 have relied on various types of optical schemes to inspect discrete items such as containers as well as web-type goods such as sheets of plastic or fabric.
Some optically-based automated inspection systems have employed a camera to image the inside of the item being inspected. However, such inspection systems did not work well with items of certain types as, for example, closures in which internal threads or other design features tended to obstruct the image of the recessed or internal surface of the closure, particularly in the vicinity of the seal.
To overcome some of the difficulties of camera imaging, some prior art inspection systems have employed multiple cameras with overlapping fields of vision. However, these systems were mechanically complicated and presented difficulties with image integration.
Such camera arrangements have been adequate for some circumstances where the recessed surface was substantially larger than the lens or where the light transmissivity of the workpiece was sufficient to illuminate the regions of concern. However, these prior art systems were not successful in all applications. For example, to provide illumination, prior art systems have sometimes used backlighting techniques. When the workpiece was opaque or sufficiently non-transmissive to light, backlighting techniques were not effective in illuminating the recessed surface. Indeed, in some applications it was found that backlighting could actually obscure the defects that were being sought.
Wide aperture or "fish eye" lenses have generally not been used in applications where the size of the lens was substantially equal to or even greater than the size of the recessed surface that was being inspected. In those applications it was found that the lens tended to block the illumination of the recessed surface. Consequently, the recessed surfaces could not be illuminated sufficiently to allow the recessed surface to be adequately imaged.
Accordingly, in the prior art there existed a need for an improved optical inspection system that could inspect recessed surfaces that had openings that were relatively small in comparison to the size of lenses that were generally used. In particular, there was a need for a system that could employ a wide angle lens to inspect such relatively small recessed surfaces.